Exploring Leflunomide’s Role in RA with Interstitial Lung Disease

Interstitial lung disease (ILD) is a hetero­geneous group of lung paren­chymal disorders that share several clinical, radiologic and histopathologic features, and are therefore grouped together.¹ ILD can occur in association with most rheumatic connective tissue diseases (CTDs), but patients with systemic sclerosis, polymyositis, dermatomyositis and rheumatoid arthritis have the highest risk of developing ILD.²

A multidisciplinary panel in 2017 reported that around 10% of patients with rheumatoid arthritis (RA) have clinical ILD. Another 30% may have subclinical ILD, indicating that about 40% of RA patients may have some form of lung involvement during the course of their disease.²

Patients may present with ILD concurrently with their RA diagnosis, develop ILD along the course of RA or, less frequently, present first with ILD without typical RA symptoms at the time of diagnosis.^1,3 The fact that several disease modifying anti-rheumatic drugs (DMARDs) have been implicated as a cause of ILD adds to the conundrum.⁴

This poses a significant challenge when choosing the most appropriate DMARD for patients with RA.

Methotrexate

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Methotrexate is the usual first-line treatment and the cornerstone of therapy for RA.⁵ It has been widely reported as a cause of ILD and acute interstitial hypersensitivity pneumo­nitis.^6-8 Lung toxicity due to metho­trexate can occur at any time during the course of treatment, which makes it difficult to assess its precise incidence.^4,9

More recent data indicate the risk of ILD with methotrexate is not as high as previously thought.¹⁰ In fact, methotrexate may not increase the frequency of ILD at all. The propensity of RA to cause ILD may be responsible for the earlier overestimates.¹⁰

On the other hand, several reports have indicated the presence of pre-existing ILD (clinical or sub-clinical) is a risk factor for hypersensitivity pneumonitis.^11-13

When ILD is not present before instituting methotrexate treatment, it is often challenging to distinguish methotrexate-induced lung toxicity from RA-associated ILD. Even though some of the reluctance to use methotrexate may be misplaced, situations with pre-existing ILD may require consideration of alternatives to methotrexate.

Leflunomide

Leflunomide is an inhibitor of pyrimidine synthesis, which is thought to be the mechanism for its anti-inflammatory and immunomodulatory properties. It is available in doses of 10 mg and 20 mg, taken as a daily tablet.

Leflunomide was first introduced in the 1990s and initial reports indicated a very low incidence of ILD.⁴

The initial concerns regarding lung toxicity with leflunomide were raised on the basis of cases reported in Japan, followed by cases from Korea and other parts of the world.^14-18

The authors of a systematic review in 2014 concluded that leflunomide can be associated with ILD in RA and that it can be potentially fatal and may present more acutely than methotrexate-induced pneumonitis.⁴

However, they do acknowledge the difficulty of distinguishing lung toxicity from leflunomide from RA-ILD. Additionally, since the majority of patients who developed lung toxicity on leflunomide in various studies have either been exposed to methotrexate in the past or had pre-existing ILD, or both, it is difficult to conclusively implicate leflunomide as a cause of lung toxicity.

Raj et al. concluded in a systematic review in 2013 that leflunomide can cause ILD and that most patients develop lung toxicity within three months of starting it.¹⁹ They note that patients with pre-existing ILD are most likely to develop lung toxicity.

In 2006, a case control study by Suissa et al. found 74 cases of serious ILD out of a total of 62,734 patients with RA who were prescribed DMARDs.¹⁸ The risk of ILD increased with the use of leflunomide.

Interestingly, in patients who had no previous exposure to methotrexate and no history of ILD, the risk associated with leflunomide was not elevated. Additionally, patients who had a prior history of ILD were twice as likely to be prescribed leflunomide than any other DMARD. Therefore, the authors conclude the reports of ILD associated with leflunomide use are likely due to channeling patients with ILD or a history of methotrexate use to treatment with leflunomide.

A meta-analysis of double-blind, rando­mized controlled trials of leflunomide vs. placebo or active comparator drugs by Conway et al. in 2016 evaluated a total of eight studies that met the inclusion criteria, and four of these had placebo comparators. Studies with fewer than 50 subjects or duration less than 12 weeks were excluded.²⁰

The results show no increase in total adverse respiratory events with leflunomide relative to comparator drugs. In fact, lefluno­mide is associated with a decreased risk of non-infectious adverse events. The authors acknowledge that in most of the individual, randomized controlled trials included, the number of patients was relatively small, thus reducing the ability to detect small increases in risk that may be clinically significant. However, the data from these trials were pooled in the meta-analysis, leading to increased power and equal distribution of bias. Although ILD can occur at any point along the course of RA and its treatment, the inclusion of randomized controlled trials at least 12 weeks or longer does counter some of the concerns raised specifically regarding leflunomide and lung toxicity.

Because RA is a systemic disease, it logically follows that control of overall disease activity should lead to a reduced risk of pulmonary involvement as well.